Widely used is a liquid crystal display device with a system in which a liquid crystal layer of twist-arranged nematic liquid crystals are interposed between two orthogonal polarizing plates and an electric field is applied in the perpendicular direction to the substrate, so-called TN mode. In the system, since the liquid crystal rises relative to the substrate at the time of black level, when viewed from an oblique direction, birefringence due to the liquid crystal molecule generates and light leakage occurs. To solve the problem, a system, in which a film in which liquid crystalline molecules are hybrid-aligned is used to optically compensate a liquid crystal cell and prevent the light leakage, is put into practical use. However, even when liquid crystalline molecules are used, it is very difficult to optically compensate a liquid crystal cell completely without problem, thereby generating such problem that graduation reversal in the bottom of a screen can not completely suppressed.
In order to solve such problem, there have been proposed and put into practical use a liquid crystal display device according to so-called in-plane switching (IPS) mode in which a lateral electric field is applied to the liquid crystal, and vertical alignment (VA) mode in which a liquid crystal having negative permittivity anisotropy is vertically aligned and alignment-divided by a protrusion or slit electrode formed in a panel. Recently, these panels are being developed not only for monitor application but also for TV application, and, concurrently, luminance of the screen has been significantly improved. Therefore, slight light leakage in a diagonally oblique incident direction at the time of black level, which was conventionally not seen as a problem in these operation modes, has come to the surface as a cause of the lowering of display quality.
As one of means for improving hue or viewing angle at black level display, arrangement of an optical compensatory material having birefringence property between a liquid crystal layer and a polarizing plate is also examined in IPS mode. For example, there is disclosed that, by arranging birefringence media having function of compensating increase and decrease in retardation of a liquid crystal layer in an inclined state while setting optical axes to be perpendicular to each other between a substrate and a polarizing plate, coloring when white level display or gray level display is looked from an oblique direction can be improved (see JP-A-9-80424). Further, there are proposed a method in which an optical compensatory film composed of a styrene-based polymer having negative intrinsic birefringence or a discotic liquid crystal compound is used (see JP-A-10-54982, JP-A-11-202323, JP-A-9-292522), a method in which a film having positive birefringence and an optical axis within the plane thereof and a film having positive birefringence and an optical axis in the normal direction thereof are combined as an optical compensatory film (see JP-A-11-133408), a method in which a biaxial optical compensatory sheet having retardation of half wavelength is used (see JP-A-11-305217), and a method in which, while using a film having negative retardation as a protective film of a polarizing plate, an optical compensatory layer having positive retardation is provided on the surface thereof (see JP-A-10-307291).
However, since most of the proposed systems are systems in which viewing angle is improved by canceling anisotropy of birefringence of liquid crystal in a liquid crystal cell, there is such problem that light leakage based on departure of cross angle of polarizing axes from orthogonality when orthogonal polarizing plates are viewed from an oblique direction can not sufficiently solved. Further, it is very difficult to optically compensate a liquid crystal cell completely without problem even in a system that is said to be capable of compensating this light leakage. Further, in an optical compensatory sheet for an IPS mode liquid crystal cell in which optical compensation is carried out by a stretched birefringence polymer film, the use of plural films is necessary. As the result, the thickness of the optical compensatory sheet increases, which is disadvantageous for thinning a display device. Further, since a sticky layer is used for laminating stretched films, the sticky film contracts due to variation of temperature and humidity to lead to generation of such defects as delamination between films and warpage.
On the other hand, a method has been examined to achieve optical compensation of IPS by combining retardation films having positive birefringence and an optical axis in the normal direction. In order to realize such retardation film, a method is known in which a liquid crystal material is aligned vertically and the alignment state is fixed (e.g., see JP-T-2000-514202 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application), JP-A-10-319408, JP-A-6-331826).
As a method for aligning a rod-shaped liquid crystal compound vertically, a method in which a vertical alignment film is employed, or a method in which a layer of vertical aligning agent (e.g., a silane-coupling agent substituted by quaternary ammonium) is formed on a substrate and a layer of a liquid crystal compound is formed on it, is proposed (e.g., Okano Mitsuharu et al. “Liquid Crystal” Applications, Baihu-kan (1985) p 61; Inamura Shohei Appl. Phys, Lett. vol. 33, No. 1978, P 1-3). However, recently, requirement for display property of a monitor has become difficult and a conventionally known anisotropic substance having been fixed by a vertical alignment system is not sufficient in micro uniformity, thus improvement of which is required.
In addition, conventional techniques developed optical compensatory sheets mainly assuming compact-sized or medium-sized liquid crystal display devices of 15 inches or less. However, recently, it is necessary also to assume liquid crystal display devices of 17 inches or more with high luminance.
It is known that improvement of leveling property is effective in order to reduce unevenness at drying. As a means for improving leveling property, a method of adding surfactant in a coating composition is proposed. This bases on such mechanism that addition of surfactant in a coating liquid lowers surface tension thereof to improve wettability to a body to be coated and diminishes or lowers variation of the surface tension in a formation process of a coated film, thereby preventing heat convection to improve uniformity of the film (see “Current Techniques of Additives for Coating” Kiryu Haruo (edition), CMC, 2001). Although the most appropriate type of surfactant varies depending on compatibility and the like with a solvent, a resin and various types of additives in an intended coating composition, in the case of solvent coating, the use of a fluorine-containing surfactant, which is soluble in a solvent and has the highest performance of reducing surface tension, is effective.
Generally, a fluorine-containing surfactant is composed of a compound having, in one molecule, a fluoroaliphatic group for realizing function of reducing surface tension and a lyophilic group for contributing affinity to various compositions for such applications as coating and molding substance when the surfactant is used as an additive. Such compound can be obtained by copolimerizing a monomer having a fluoroaliphatic group and a monomer having a lyophilic group.
Typical examples of the monomer having a lyophilic group to be copolymerized with a monomer having a fluoroaliphatic group include poly(oxyalkylene) acrylate and poly(oxyalkylene) methacrylate. Further, JP-A-2004-333852 discloses an optical film in which both of improvement of unevenness in thickness caused by drying air and optical performance are satisfied by an ω-H type fluorine-containing polymer surfactant.
However, when a conventional fluorine-containing surfactant is used, although drying marks and unevenness in thickness caused by drying air is improved, there is such problem as generation of coating unevenness to lower grade of the coated optical compensatory film. Further, JP-A-10-309455 discloses a fluorine-containing surfactant containing an ethylenic unsaturated monomer having a branched aliphatic hydrocarbon group, but it does not describe about an optical compensatory film and does not clarify the effect thereof.